Finite Grid Operation

1. FINITE BUS OPERATION
OBJECTIVES
1. Define a finite bus.
2. Explain how a generator behaves when it is loaded onto a finite bus. The actions of the
governor and AVR must also be described.
3. Explain the safety precautions that must be observed when generators are
(a) excited,
(b) loaded
(a) unloaded.
INTRODUCTION
The previous lesson considered generator loading when the terminal
voltage and frequency were fixed by the infinite bus. This section examines the behavior
of a generator when it is loaded onto a finite bus.
By definition a finite bus does not operate with constant voltage and frequency.
It is normally assumed that when a generator has a capacity of greater than 5% of the
system size, then with respect to this generator, the system does not behave as an infinite
bus. For example, when an 800 MW generator is loaded onto a grid having a capacity of
10,000 MW, the system voltage and frequency can vary and the system will behave as
finite bus. This lesson also considers the action of the governor controlling the frequency
and the automatic voltage regulator (AVR), keeping the terminal voltage constant.
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2. GOVERNOR ACTION
Governor Characteristics
Governors have a characteristic called Droop. When the turbine speed or
generator frequency decreases, the governor will signal the governor valves to open. This
will allow more steam to enter the turbine and the turbine will produce more power.
Figure 1: Diagram showing droop characteristic of a turbine governor.
Figure 1 shows characteristic lines for a governor having a 4% droop. The
following examples will illustrate the significance of these droop lines.
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3. Example 1:
The turbine is operating on curve 1 at point “A”, producing 100% load at 100%
speed. If for any reason the load frequency rises by 2%, the turbine—generator speed will
also rise by 2%. The governor will sense this rise and the governor will partly close the
throttle valves. The turbine will then operate at point “B” where it will give 50% power
output. A 4% system frequency speed rise to 104% will, by governor action,
automatically fully close the governor valves. The turbine will now operate at point “C”.
Example 2:
The turbine is operating at point “D” on curve 2 producing 50% load at 100%
speed. If the operator wishes to decrease output from 50% load to 0% load, he will lower
the governor droop line by operating the speeder gear. In doing this, the operator will be
changing the characteristic from the #2 to the
#3 line.
When the turbine is at 0% load and 100% speed, it will be operating on #3 line at
point “E”. In this example, it is assumed that the system frequency remains the same (60
Hz) and the output from this generator drops to 0% load.
Frequency Variation, Output Voltage and Power
The speed of a turbine generator, when operating on a given 4% droop line, can
only vary a maximum of 4%, between no load and full load. The output voltage VT for a
given level of excitation will also vary by only 4%, (VT proportional to speed). When the
voltage VT increases, the load consumes more active power output (P) from the generator.
(It is assumed there is sufficient steam available..) When VT falls the load consumes less
active power output. This is because P = VT
2
/R. For example, if the speed and hence
output voltage rises by 1%, then the power produced by the generator and consumed by
the load will rise by [l.0l2
= 1.02] approximately 2%. This example shows, that for
normal changes (less than 1%) in frequency from the standard 60 Hz, the change in active
power consumed by a load (and given out from a generator) is small. In this case, the
excitation is assumed to be constant. In practice, a voltage regulator keeps the terminal
voltage constant.
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4. Summary
Turbine-generators, having governors with 4% droop characteristic will:
(a) for a given governor setting (droop line), decrease load if the system frequency rises.
Conversely, if the frequency falls, the output will attempt to rise. The output is limited by
power of prime mover and heat source.
(b) for a given speed (frequency), decrease load (MW output) if the setting (droop line)
is lowered, conversely, if the setting is raised, the output will increase.
(c) require operator action to raise or lower the droop line.
AVR ACTION
Operation With The AVR In Service
Automatic voltage regulators (AVR’s), control the terminal voltage of a generator
by altering the level of excitation. An AVR, at a given voltage setting, keeps the terminal
voltage constant from no load to full load. Consequently, apart from the very short time,
typically .25 sec, required for the AVR to respond and correct a change in terminal
voltage, the terminal voltage of an AVR controlled generator will be constant.
Operation With the AVR Out of Service
If a generator is operated on the grid system with its AVR on “manual” instead of
“auto”, due to an AVR fault for example, dangerous operating condition can occur.
Consider the following three cases:
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5. Figure 2: Generator feeding a finite bus grid load: Generator output increased,
excitation constant.
Case 1
The generator (with fixed excitation) is on no load connected to a finite bus. the MW load
is increased. See Figure 2.
(a) The increase in load will produce an increase in Ia and a corresponding increase in
armature reaction.
(b) The rotor flux is constant due to fixed excitation. Due to increased armature reaction
because of increased load, it follows that the terminal voltage will decrease.
(a) The increase in armature reaction, with a constant rotor flux, will cause
the load angle δ to increase. The load angle can easily be made to exceed the 90° steady
state stability limit. Pole slipping may result.
(d) The fall in terminal voltage will reduce the finite bus voltage. The amount by which
the system voltage will fall is largely dependent upon other generators on the system
having AVRs which have the ability to correct this voltage decrease. It will also depend
on the relative transmission line lengths between generators and the load. Figure 2 shows
the operational conditions. This situation is most likely to occur where the generation is
remote from the load. An example of this is at Bruce where the main loads are distant
from the generating station.

6. —5—
Case 2
The generator (with fixed excitation) is on full load supplying a finite bus. the generator
output is then reduced. The following occurs:
(a) The decrease in generator output will produce a decrease in load current and a
corresponding decrease in armature reaction.
(b) Load angle δ decreases. This is not dangerous as the generator is more stable; it is
more secure on the system.
(c) As the armature reaction decreases, the generator becomes overexcited and the
terminal voltage will rise. The amount that the system voltage rises is again dependent
upon other generators on the system having AVRs with the ability to correct this voltage
change. It will also depend upon line lengths and the load. Figure 3 shows the operational
conditions.
Figure 3: Generator feeding finite bus grid load. Generator output decreased:
excitation constant.
Case 3
A generator’s main output breaker may trip, due faults which can occur at any instant.
The trip can occur when the level of excitation is above that required at no load, When
this occurs, the field current must be immediately reduced. With the AVR out of service,
this reduction must be done manually. Failure to do this reduction immediately will cause
the output voltage to greatly exceed its rated value and damage may result. This damage
can take the form of insulation failure or overfluxing which is described in the next
lesson.

7. —6—